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D4 stellar processes


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stellar processes

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D4 stellar processes

  1. 1. Astro - HL D4 stellar processes
  2. 2. The Perfect Storm A small region in the Swan Nebula, 5,500 ly away, described as 'a bubbly ocean of hydrogen and small amounts of oxygen, sulphur and other elements'.
  3. 3. Star Formation - recap Gravity begins to pull the gas and dust together. They lose gravitational potential energy… this is converted into kinetic energy. The temperature increases.
  4. 4. Jeans criterion In order for stars to form a portion of the molecular cloud of particles must first collapse to become the kernel. This gravitational collapse only occurs in certain situations. James Jeans showed that a cloud of given radius and temperature, has a critical mass (the Jeans mass) which if exceeded, will cause the cloud to become unstable and collapse.
  5. 5. Jeans mass The Jeans mass depends on the radius of the cloud, temperature, average mass of the particles in the cloud MJ = 3kTR / 2Gm
  6. 6. Star Formation Protostar High temperature leads to ionisation of elements. E-M energy is emitted. The star can have considerable Luminosity, eg 5000 times the surface area and 100 times as Luminous as our Sun. Temperature continues to increase… Electrons stripped from the atoms in the core. A plasma is formed.
  7. 7. Star Formation Main Sequence Star- Nuclear Fusion starts up. Temperatures now high enough to fuse Hydrogen into Helium. Gravitational contraction will now stop as the Fusion process will offset the contraction. “Hydrostatic Equilibrium”
  8. 8. You are not Required to Memorize these Reactions
  9. 9. Leaving the main sequence A main sequence star has THREE main layers:  Helium core (inner layer): Releases energy as it contracts. Fusion shell: Releases energy as it fuses hydrogen into helium. Hydrogen envelope (outer layer): Absorbs energy, and expands greatly in size. These swollen stars, no longer on the main sequence, are now giants (if M < 8 Msun) or supergiants (if M > 8 Msun).
  10. 10. stars with M > 0.4 Msun  Can fuse helium into carbon by the triple alpha process. This fusion process combines three helium nuclei, or alpha particles, into one carbon nucleus. Step one: 4 He + 4 He --> 8 Be Step two: 8 Be + 4 He --> 12 C + � Optional additional reaction: 12 C + 4 He --> 16 O + �
  11. 11. Stars with M > 4 Msun Now the giants have FOUR main layers:  Carbon (+oxygen) core: Release energy as it contracts. Helium fusion shell: Releases energy by fusing helium into carbon (+oxygen) Hydrogen fusion shell: Releases energy by fusing hydrogen into helium Hydrogen envelope: Still has very large radius
  12. 12. Once the central temperature T > 600,000,000 Kelvin. Carbon & oxygen can fuse into heavier elements, such as silicon, sulfur, and iron – for the star this is a new energy source. Iron is the end of the line.
  13. 13. Mass – Luminosity Relationship There is a relationship between the luminosity of a star and its mass L = M3.5 Where L is luminosity, M is mass in solar units and applies to all main sequence stars The power (3.5) can be any value between 3 and 4 as it is itself mass dependant.
  14. 14. It is reasonable to assume that the age of the star is related to its mass. So that luminosity is determined by energy release/time, and that energy released is related to mass so, L α M/T So, combining the two relations gives T α M-2.5
  15. 15. Question If a star is twice the mass of the sun, estimate its lifetime as a main sequence star. Given the lifetime of the sun is 9.4 x 109 years.
  16. 16. Question-solution If a star is twice the mass of the sun, estimate its lifetime as a main sequence star. Given the lifetime of the sun is 9.4 x 109 years. (Ms/M)2.5 = T/Ts T = (½)2.5 x 9.4 x 109 = 1.7 x 109
  17. 17. The Supergiants
  18. 18. The Supergiants Iron cannot undergo fusion due to its very high coulombic repulsion (26 protons).It would need astronomical temperatures. The star has reached a critical state. The star will once again begin to collapse into the core. But no more fusion will take place to counteract the gravitational collapse. Incredibly high temperatures lead to the combining of electrons and protons.
  19. 19. The Supergiants Neutrons and neutrinos are formed in large quantities. High energy neutrinos form an outward pressure wave. This wave hurtles outward. This shock wave rips the outer layers off of the star. The inner core is now exposed. Huge amount of radiation floods into space.
  20. 20. Supernova The star has become a Supernova. The luminosity for a brief moment is greater than the whole luminosity of a galaxy (1 billion stars)! 96% of the stars mass is lost to space.
  21. 21. Mass determines star path
  22. 22. Neutron capture There are two forms of neutron capture that can facilitate stars fusing elements above Fe. S-process (slow capture): formation of copper, silver, gold and lead upto to bismuth209 R-process (rapid capture): elements above bismuth 209 occur only in a supernova explosion. A large flux of energetic neutrons is produced and nuclei bombarded by these neutrons build up mass one unit at a time.
  23. 23. Type Ia and IIa supernova
  24. 24. Type Ia The observed spectra over time show sharp maxima and then die away smoothly and gradually. The initiation is the detonation of a carbon white dwarf when it collapses under the pressure of electron degeneracy. It is assumed that the white dwarf exceeds the Chandrasekhar limit of 1.4 solar masses. The spectra of Type I supernovae are hydrogen poor is consistent with this model, since the white dwarf has almost no hydrogen. They can be used as standard candles.
  25. 25. Type IIa Are implosion-explosion events of a massive star. They have a characteristic plateau in the observed spectra over time, a few months after initiation. It is assumed that the energy comes from the expansion and cooling of the star's outer envelope as it is blown away into space. There are strong hydrogen and helium spectra for the Type II supernovae.
  26. 26. Light curves for the supernova
  27. 27. Credits Hyperphysics, Windows to the universe /Nuclear_Reactions/Fusion/Fusion_in_stars/ncaptur e.html Phys 306 s.html